2o8 ROOTS OF WATER PLANTS [ch. 



character of the growths which spring from the stalk; for these 

 are neither leaves nor stalk ^ . " We have already alluded to the thal- 

 loid roots of the Podostemaceae, which also serve for assimilation . 

 Like the stems of aquatics, the roots show certain anatomical 

 divergences from those of land plants^. Root hairs are occasion- 

 ally absent, e.g. hemna trisulca. The roots of Elodea bear no 

 absorbent hairs so long as they are immersed in water, but they 

 develop them freely on entering the soiF. In other hydrophytes, 

 e.g. Hydrocharis^ the root hairs are unusually long. It is rather 

 curious that in the roots of water plants the piliferous layer, 

 and the layer immediately below it, are often cuticularised. The 

 ] aerating system, which occurs in the primary cortex, or as a 

 secondary formation, has been dealt with in Chapter xiv. 



As in the case of submerged stems, the vascular system of the 



roots tends to be very much reduced. The simplest root among 



Dicotyledonous water plants is that of Callitriche stagnalis (Fig. 



138), which has two protoxylems each consisting of a single 



tracheid separated by a single median metaxylem element. 



This simple xylem group is flanked on either side by a single 



sieve-tube with companion-cells. In certain Monocotyledons, 



/ a still more extreme degree of simplification is reached. Vallis- 



\ neria spiralis (Fig. 139), for instance, has merely a central 



I channel, corresponding to the central vessel of other forms, 



! surrounded by a ring of cells, three of which are apparently 



sieve-tubes, each accompanied by a companion-cell. Naias, 



again, has a root of a very simple type, in which the phloem is 



more conspicuously developed than the xylem'* (Fig. 140)- The 



reduction series in the roots of the Potamogetons is illustrated 



in Fig. 41, p. 6^. 



Plasticity is certainly a marked feature of the roots of water 

 plants, for though they have to some extent given up the work 

 of absorption, they have assumed and developed various other 

 functions to which their terrestrial ancestors must have been 

 comparative strangers. 



1 Theophrastus (Hort) (191 6). 2 Schenck, H. (1886). 



3 Snell, K. (1908). 4 Sauvageau, C. (1889I). 



